WASHINGTON – Investigators looking into the deadly crash of two Metro transit trains focused yesterday on why a computerized system failed to halt an oncoming train, even though there is evidence that the operator tried to slow it.

At the time of the crash, the train was operating in automatic mode, meaning it was controlled primarily by computer. In that mode, the operator's main job is to open and close the doors and respond to emergencies.

Debbie Hersman, an investigator with the National Transportation Safety Board, said it was unclear whether the emergency brake was engaged when Monday's crash occurred. But the mushroom-shaped button that activates it was found pushed down in the operator's compartment.

Hersman said it wasn't clear when the button was pressed or how it got that way. But she also said there was evidence of braking on the train's rotors, indicating it was likely that the operator tried to slow down.

In addition, Metro sources said, the first two cars of that train were two months overdue for scheduled maintenance of some braking components.

Taken together, experts say these facts point to several possible scenarios: The operator activated the brake too late; the computers that are supposed to stop a train from getting too close to another train faltered; the train's brakes failed; or some combination of those.

The train plowed into a stopped train ahead of it at the height of the Monday evening rush hour, killing nine people and injuring more than 70 in the deadliest accident in the 30-year history of the Metro.

Crews yesterday were pulling apart the wreckage and searching for bodies. Authorities also worked to determine why the train's safeguards apparently did not kick in.

“That train was never supposed to get closer than 1,200 feet, period,” said Jackie Jeter, president of a union that represents Metro workers.

All Metro trains were running on manual control yesterday as a precaution against computer malfunction.

The cars in the moving train were some of the oldest in the transit network, dating to 1974.

Federal officials had sought to phase out the aging fleet because of safety concerns, but the transit system kept the old trains running, saying it lacked the nearly $1 billion required for new cars.

Hersman told The Associated Press that the NTSB had warned in 2006 that the old fleet should be replaced or retrofitted to make it better able to survive a crash.

Neither was done, she said, which the NTSB considered “unacceptable.”

Metro General Manager John Catoe said the agency expected to receive proposals “over the next month or so” to replace the old cars, but new trains were still years away from being installed. He insisted the existing cars were safe.

This isn't the first time that Metro's automated system has been questioned.

In June 2005, Metro had a close call because of signal troubles in a tunnel under the Potomac River. A conductor noticed he was getting too close to the train ahead of him even though the system indicated the track was clear. He hit the emergency brake in time, as did the operator of a train behind him.